WHILE PERFORMING A TOUCHDOWN AUTOROTATION AT ABOUT 3 FT AGL, THE STUDENT INITIATED CUSHIONING COLLECTIVE PITCH & LEVELED THE HELICOPTER. AT APRX 1 FT AGL, THE STUDENT APPLIED AFT CYCLIC CONTROL & THE HELICOPTER TOUCHED DOWN ON THE AFT PORTION OF THE SKIDS. THE INSTRUCTOR & STUDENT APPLIED FWD CYCLIC CONTROL & THE HELICOPTER ROCKED BACK & FORTH SEVERAL TIMES BEFORE COMING TO REST. NO CONTROL INPUT WAS MADE BY THE CFI BEFORE IMPACT.